Yes Rajnath knew Shakshi was married before.
The tape is the mystery yet to be solved.
Yes, Raima knows about Shakshi's past from Rajnath.
Rajnath did not accept Shakshi while she was married because Shakshi killed her husband and then married Rajnath.
I doubt Shakshi or Rajnath liked or loved each other. They married either because of blackmail on each other or to join forces to gain Goenka Empire.
Yes, Shakshi definitely married for Power and Money and Rajnath most probably to save his neck. These two are Birds of the Same Feather and so have had to stay together to keep the Power between them.